In recent years, development of high-density display panels has been based importantly on picture display apparatus using liquid crystals. To perform high-density display by use of such display elements which have relatively poor time division properties, the following two approaches have generally been adopted. The first approach involves increasing the display density by means of multiple wiring and the like, setting the time-division degree at an appropriate value, and utilizes a multiple matrix system. The second approach involves forming active elements such as transistors, non-linear resistances, and the like as switches on the panel to accumulate voltage for each picture element, and utilizes an active matrix system. With the first aproach, panel manufacturing is relatively easy but the increase in multiplexity of wiring is limiting, so that the display density cannot be readily increased. With the second approach, the display density can be incresaed considerably but it is difficult to manufacture and to set high-performance switches able to accumulate charge sufficiently in the panel, while retaining high density with few defects. An example of a panel of this active matrix type is disclosed in Japanese Laid-Open Patent Publication No. 15449/1984.
FIG. 1 shows a schematic circuit of a portion of the display disclosed in the above-mentioned Laid-Open Patent Publication.
Such a display includes a matrix on one surface of which is a plurality of row electrodes 21, also sometimes termed the scanning electrodes, of which three are shown, designated S1, S2 and S3 and on the opposite surface of which is a plurality of column electrodes 23, also sometimes known as the data electrodes, of which three are shown, designated D1, D2 and D3. At the crosspoints corresponding to the points at which these electrodes would intersect if in a common plane rather than on spaced planes, are formed the display elements, each corresponding to a picture element. At each crosspoint serially connected between the opposed electrodes is included a separate liquid crystal element 24 and a separate parallel pair of oppositely-poled diodes 22, which serve as a non-linear resistance.
As depicted, the row and column electrodes are orthogonal to form a two-dimensional array of picture elements. Typically the set of electrodes on the viewing side of the display are transparent for light transmission. Liquid crystal elements typically of thickness of about ten microns are included between the sets of electrodes.
The typical structure of one diode of the diode pair 22 included in FIG. 1 is shown in section in FIG. 2. Actually, to increase the threshold voltage of each of these diodes, each effectively is a series connection of two PIN sets of layers.
To this end, the diode is formed on a transparent insulating substrate 1, typically of glass. The substrate directly supports transparent electrode 2, an extension of which becomes electrodes 21 shown in FIG. 1, and transparent electrode 3, an extension of which makes connection to one side of the liquid crystal element 24, as shown in FIG. 1. Each of electrodes 2 and 3 typically is of indium-doped tin oxide (ITO). The series duo of PIN layers is formed over electrode 2. It comprises an electrode 41, p-type layer 51, intrinsic or very lightly doped layer 52, and n-type layer 53. Each of layers 51, 52 and 53 is of amorphous silicon (a-silicon) formed in known fashion by a glow discharge decomposition process and typical thicknesses of layers 51, 52 and 53 are 500 Angstroms, 0.5 micron and 500 Angstroms, respectively. Over layer 53 is formed a second electrode 42 over which are deposited a second set of layers 51, 52, 53 as before and a third electrode 43 is deposited over layer 53. Each of electrode layers 41, 42 and 43 typically is of chromium deposited by electron beam vapor deposition, or sputtering, and has a thickness of about 1000 Angstroms. These layers typically are patterned to the desired form by photoetching techniques.
There results between electrodes 41 and 43 a pair of PIN laminates. The resulting structure has a threshold voltage betwen electrodes 41 and 43 twice that a single PIN laminate would have and such higher threshold is desirable for improved control of the liquid crystal element with which it is to be associated.
Next, there is formed an insulating film 6, for example of silicon nitride deposited by glow discharge decomposition of a silane and ammonia mixture in known fashion. The film is patterned as desired, typically by photoetching. The film is provided with an opening 7 where it overlies the central portion of electrode 43 and an aluminum layer 8 is deposited over the film 6, typically by vapor deposition or sputtering to contact electrode 43 by way of the opening 7 in the film 6. Layer 8 has an extension which makes electrical connection to electrode 3. The layer 8 is patterned, typically by photoetching, not to extend so far as to contact electrode 2. It also does not wrap completely around the edges of the laminate, since this was thought to be unnecessary.
Moreover, although not shown in FIG. 2, a similar structure would be formed over a portion of electrode 3, and the final aluminum layer 8 in this case would make electrical connection selectively only to electrode 2 whereby there would be formed between electrodes 2 and 3 a parallel pair of diodes oppositely poled, each of the diodes comprising a series of two PIN elements of the kind shown in FIG. 2.
FIG. 3 shows the typical current-voltage characteristics of the parallel pair of oppositely-poled diodes and the solid line shows the characteristics in the dark of a good pair. However, when the pair was illuminated with ambient light at low voltages the broken-line characteristic resulted. This showed high current even at low bias voltages. When this current is high, the ability of the diode pair to accumulate voltage is lowered and this impairs the quality of the display considerably. What was surprising was how sensitive the quality of the display was to ambient light incident on the diode.
The object of the invention is a diode pair of the general kind described for use in a liquid crystal display and which has a low current-low voltage characteristic both in the light and in the dark.